3He in the Milky Way Interstellar Medium: Abundance Determinations

We are making precise determinations of the abundance of the light isotope of helium, 3He. The 3He abundance in Milky Way sources provides strong constraints for theories of stellar evolution, chemical evolution, and cosmology. Here we report on new 3He+/H+ abundance determinations for a sample of 21 Galactic H II regions observed for the past 14 yr with the NRAO 140 Foot telescope. The abundances are based on improved 3He+ observations, as well as new models for source density structure. The models are constrained by the observed 3He+ emission, the strength of the H and 4He recombination lines, and the continuum emission at 3.5 cm wavelength. Density structure serves only to increase the actual 3He+/H+ abundance ratio over that derived using a uniform density sphere assumption. The density structure models allow one to identify sources that are "simple" in that their structure corrections are negligible. These objects provide a pool of H II regions that can in principle yield accurate 3He+/H+ abundances. For the seven simple H II regions in our sample there is no evidence for any systematic trend of the 3He+/H+ abundance with any other parameter. For these sources, we derive an average abundance ratio by number of 3He+/H+=(1.6±0.5)×10−5 (statistical error).

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